1. Field of the Invention
This invention relates generally to optical systems and more particularly to an optical system which uses a pivoting hologram to adjust beam position.
2. Description of the Prior Art
Optical data storage systems typically have an optical data storage disk medium which stores data on concentric or spiral tracks. A fixed optical system (FOS) contains a laser for generating a laser beam to be directed to the disk and sensors for detecting reflected light from the disk. The data is accessed by positioning a movable optical head to a position proximate the desired track. The head is moved by a radial linear motor, otherwise known as a coarse tracking actuator.
The optical head typically contains a rotatable mirror for directing the laser from the FOS to the disk and an objective lens for focusing the beam to the disk. The objective lens is attached to a focus actuator which moves the lens in order to keep the beam properly focused on the disk. A fine tracking actuator in the optical disk moves the rotatable mirror such that the beam of light always remains exactly on the desired track. Examples of these types of optical systems include U.S. Pat. No. 4,466,088, issued Aug. 14, 1984, to Trethewey; U.S. Pat. No. 4,564,757, issued Jan. 14, 1986, to Labudde et al; U.S. Pat. No. 4,744,071, issued May 10, 1988, to Bricot et al; U.S. Pat. No. 4,969,137, issued Nov. 6, 1990, to Sugiyama et al; Japanese Patent Application JP56-83850, published Jul. 8, 1981, by Nagashima et al; and Japanese Patent Application JP02-216625, published Aug. 29, 1990, by Maeda et al.
In each of these systems, the mirror is rotated such that the beam is displaced and fine tracking on the data track is achieved. A problem with these schemes is that the aperture of the objective lens in the optical head is relatively small and the beam displacements caused by the rotatable mirror cause a large fraction of the beam to miss the aperture of the objective lens. This causes a large variation in the strength of the data signal which can result in misreading of the data. Also, as the beam moves away from the center aperture, beam distortions and aberrations may occur.
U.S. Pat. No. 4,782,474, issued Nov. 1, 1988, to Arai et al shows one possible solution to this problem. Here the objective lens is mounted to an additional actuator which moves the lens laterally as the rotatable mirror displaces the beam. By coordinating the movement of the mirror and the lens, the beam will always pass through the center of the lens. The problem with this scheme is that it adds additional components, circuitry, complexity and cost to the system.
What is needed is a simple and inexpensive way to achieve fine tracking without displacing the beam from the lens aperture.